1. Field of the Invention
This invention relates to liquified gas storage tanks, and more particularly to a new and improved overfill protection system mounted inside the storage tank.
2. Description of the Prior Art
In recent times, liquified gases have become popular for use as a fuel in many industrial and commercial applications. Liquified gases are extremely cold and as such thermal storage tanks have been developed to permit safe and economical use of liquified gases as fuels. These tanks are generally known as dewar tanks.
One of the important and relatively new uses of liquified gas includes use as fuel for powering internal combustion engines in both commercial and private vehicles. Typically, liquid natural gas, LNG, is the preferred gas for this purpose. The development of LNG as a fuel has been concentrated in the area of commercial vehicles, such as tractor trailers, delivery trucks, and mass transit vehicles.
One of the major problems encountered in using LNG as a fuel is the refilling of the fuel tanks after they have been depleted. There are several concerns associated with refilling LNG storage tanks on commercial vehicles. Some of these include filling the tanks to the appropriate level to allow a sufficient holding time between when the tank is filled and the pressure in the tank reaches the relief pressure, minimizing the manpower necessary to fill the tanks, and minimization or elimination of the venting of the LNG to the atmosphere during the filling process.
The holding time of a tank is the difference in time between when the tank is filled and when the tank reaches the set pressure of the relief valves. The holding time is important because an LNG tank is not always used immediately after being filled, and must be able to sit for a given time until it is needed. Typically, the full tank is not interfaced with temperature and pressure control apparatus during the holding time. Therefore, the contents of the tank are subject to the external ambient conditions during the holding time. To allow for a given holding time, there must be a certain amount of gaseous volume in the tank, depending on the physical conditions inside and outside the tank, to allow the pressure inside the tank to rise without reaching the predetermined relief pressure. In addition, it is desirable to assure that only gas will vent if the relief valves open.
An accurate and reliable system for use in determining when an LNG tank has been filled to the appropriate level is an important component in addressing the problems of hold time and venting. When filling a tank with LNG, a balance must be struck between filling the tank to a liquid level such that there is an appropriate amount of vapor space to be pressurized as the tank absorbs heat from the environment, and the need to provide a sufficient amount of vapor space to allow the tank to have a sufficient holding time.
Also of concern is the need to have a tank that can be filled sufficiently full so as to not waste valuable space and weight on the vehicle. The vapor space must be adequate to allow the tank to absorb energy from its surroundings without reaching the relief pressure before the predetermined hold time has expired. Typically these tanks have a minimum holding time of between seven and eight days, and a fill pressure between 20 and 80 psi, with a relief pressure of approximately 250 psi.
If the tank is overfilled, there is not enough vapor space left in the tank to allow the tank pressure to increase without venting due to heat absorption from the surroundings. This situation would cause the pressure inside the tank to reach the relief pressure before the predetermined hold time has expired, thus causing LNG to vent into the atmosphere through a the relief valve.
Venting LNG to the atmosphere has known adverse environmental impacts, and is a highly regulated activity. Venting natural gas to the atmosphere carries with it considerable risk both to the environment and to the safety of the tank surroundings. On the other hand, if the tank is underfilled, the operator is then not able to obtain the maximum benefit of a full tank. The same safety and environmental concerns do not accompany underfilling an LNG tank.
Accurate, repeatable, and convenient apparatus that meets the above requirements are also of great importance economically. A fill procedure that is nearly automatic and requires a minimum amount of labor is desirable from an economic standpoint. An overfill protection apparatus that allows for accurate and repeatable results would make the use of LNG more economically feasible, creating a favorable incentive to the LNG industry to further develop LNG as a fuel.
Several prior art fill systems have been developed for use with LNG. One of the most basic is the use of differential pressure to measure the volume level of liquid in a tank in order to determine when the tank is sufficiently full. Differential pressure measurement is typically used in large stationary cryogenic tanks. Differential pressure measurement is accomplished with one gage that is capable of measuring the differential pressure across a diaphragm. This system works fairly well for large stationary tanks.
However, LNG is very light, and it has been found uneconomical to develop a sensor system that is sufficiently accurate to measure the slight changes in pressure in a tank of the size used on a commercial vehicle, and also make that measurement apparatus strong enough to withstand the impact, jarring and general abuse it would encounter when being used on a commercial vehicle. These sensors are very delicate and have been found to be inaccurate and even fail during use in the vibrating and abusive world of an over-the-road vehicle.
Another apparatus used to measure the level of LNG during the filling of a tank is a full tri-cock. The full tri-cock is simply a dip tube that spurts liquid when the liquid natural gas reaches the end of the tube inside the tank. This apparatus obviously is not adequate to control the venting of liquid natural gas to the atmosphere. It is also labor intensive.
Other devices used to measure the level of LNG in a tank during filling include point sensors, which are simply sensors located along a vertical rake at discreet points in the tank. As the sensors come into contact with the LNG, the sensors send a signal to a receiver indicating such. This apparatus requires monitoring during fill-up, and also is fairly expensive and susceptible to damage during use.
The problems of venting LNG to the atmosphere during the fill procedure, striking the balance between filling the tank too full and not filling it adequately, and providing a repeatable and accurate fill level have not been successfully resolved in the art. It is to overcome these shortcomings in the prior art that the present invention was developed.